CAD Repair for CFD Analysis of Turbomachinery
Recently there has been great interest by design engineers/ CFD analysts/ researchers worldwide to develop greater insights in the design and performance analysis of Turbiomachinery as they form an important component in major industrial applications. The blog aims to provide an overview of Turbomachinery and the essential CAD cleanup steps while meshing turbomachines for CFD analysis with the help of a software video demonstration on a commercial tool.
Introduction to Turbomachinery
The earliest developments of Turbomachinery is attributed to Hero (or Heron) of Alexandria (c. 10–70 AD) an ancient Greek mathematician and engineer who was the first to harness wind power on land with his invention of a windwheel and also in his book Pneumatics, described a device known as Aeolipile that worked on reaction principle. The modern days definition of Turbomachinery is as, "a device in which energy transfer occurs between a flowing fluid and a rotating element due to dynamic action resulting in a change in pressure and momentum of the fluid". The fundamental laws governing turbomachineries are Newton’s 2nd law of motion and Euler’s energy equation. The analytical calculations used for solving turbomachinery problems are based on Euler’s energy equation that relates the power added to or removed from the flow, to characteristics of a rotating blade row. The equation is based on the concepts of conservation of angular momentum and conservation of energy.
The principal components of a turbomachinery are:
- Rotating element carrying vanes operating in a stream of fluid,
- Stationary element or elements which generally act as guide vanes or passages for the proper control of flow direction and the energy conversion process,
- Input and/or an output shaft, and
As a turbomachinery exchanges energy with a continuously flowing fluid and rotating blades, mechanical energy transfer occurs into or out of the turbo-machine usually in steady flow. Turbo-machines include all the machine types that produce large-scale power and a head or pressure, such as centrifugal pumps and compressors.
Classification of turbomachines:
There are three major basis of classification of turbomachinery as listed:
1. Based on applications:
- Power absorbing turbomachines. Examples- pumps, compressors etc.
- Power generating turbomachines. Examples- steam turbines, gas turbines and hydraulic turbines.
- Power transmitting turbomachines. Examples- clutch plate gear drive etc.
2. Based on fluids handled:
- Compressible fluids. Examples-compressors, steam and gas turbines, fans, blowers, propellers etc.
- Incompressible fluids. Examples-hydraulic pumps and turbines.
3. Based on the direction of flow:
- Axial flow turbomachines. Examples-fans, propellers.
- Radial flow turbomachines. Examples-centrifugal pumps and compressors.
- Mixed flow turbomachines. Examples-Kaplan turbine.
CFD assisted design of Turbomachinery:
The conventional design process for any turbomachinery equipment was based on application of empirical design methods wherein theoretical empirical configurations were used i.e. the velocity triangle theory was used. Design of turbomachinery in general involves large number of variables and different design configurations to meet the requirements. Parametric study of all these design forms a part of conceptual design process for any turbomachinery equipment. Nowadays CFD has been increasingly used to successfully design turbomachinery as well as carry out analysis and validation requirements for existing design. Thus CFD is used in both conceptual design process and after design process in order to validate or rectify a particular design. The typical design cycle for CFD analysis constitutes of the series of steps as shown in the image below.
CFD can give important insights in turbomachinery analysis like:
- performance prediction of existing design,
- internal flow visualization in the form of pressure distribution,
- flow distribution,
- flow path through the impeller,
- diffuser and other components.
Both qualitative and quantitative analysis of performances can be obtained through simulations and also investigation of the existing design can be done where rectification can be done in which there are inefficiencies.
Meshing for CFD analysis of turbomachinery:
Meshing rotating machines/ turbomachines for CFD simulation is considered as, one of the challenging and complex jobs in CFD community. In this post, we will discuss the complete pre-processing method for turbo-machines. The objective of this discussion is to give insights of meshing strategy for turbo-machines.
First step in pre-processing is CAD repairing or CAD clean-up. This is nothing but preparing the geometry for meshing. This involves:
1. Importing Geometry :
Geometry from CAD software can be imported into the meshing software through,
- Native file format from Pro/E, CATIA, SolidWorks, etc....
- Neutral file format – IGES, STEP, ParaSolid, etc....
It is a good practice to import the geometry in 'mm'. This avoids tolerance error while defining very low ‘first cell height’ (discussed on the meshing section of this post) for meshing.
2. CFD Domain Extraction :
The geometry which we imported from CAD software packages are production models and cannot be used directly for meshing. The CFD domain needs to be separated or extracted from the imported production CAD model. In other words, only surfaces (if thermal analysis is not of interest) those are in direct contact with the working fluid need to be extracted from the CAD model.
3. Water Tight Geometry :
The very important requirement of any meshing software is a closed domain or water tight geometry. This involves:
- Boundary surfaces – create Inlet and Outlet surfaces
- CAD import errors – close small gaps between surfaces, create missing surfaces, trim surface parts going out of the domain
4. Approximating Geometry :
Any feature with intricate detail which have negligible impact on results, are removed or approximated for the ease of meshing. Below is the list of such features
- Fillets and chamfers
- Bolts, nuts, washers, threads and gaskets
- Holes, gaps and small steps
- Numbering, codes and lettering
Before simplifying any geometric feature, its impact on the physics should be understood. For example we cannot approximate the gaps between the blade tip and shroud as it may cause secondary flow, fillets on the suction side of the vane might cause flow separation which will have a considerable impact on the results.
5. Need for Interface:
Usually CFD analysis of rotating machines involves flow through domain which contain moving (rotating) and stationary components. And if the walls of the stationary parts can be described by surface of revolution, then the problem can be solved using single reference frame (SRF) approach. In this approach, the equations of the fluid flow are solved in moving reference frame which makes the solution steady with respect to the moving reference frame. Single reference frame (SRF) approach doesn't require any interface as it assumes a single fluid domain rotating at constant speed with respect to specified axis.
When the rotating machinery problem involves stationary components (w.r.t fixed frame) which cannot be described by surface of revolution (like the baffles), Single reference frame (SRF) is not valid. Systems like these are solved by dividing the domain into multiple fluid zones i.e, rotating and stationary, with an interface separating them.
6. Shape and Location of the Interface:
The interface separating rotating and stationary domain must be a surface of revolution with respect to axis of rotation of the rotating zone. The interface should be placed such that there are no stationary components which are not a surface of revolution, inside the rotating zone. When the gap between the rotating blades and stationary casing is small, the interface is placed at the middle.
7. Surfaces for Post-Processing:
Though post-processing tools allow us to create planes or surfaces on which contours and vectors are plotted, it is always a good practice to have this surfaces at geometry level and get it meshed, especially when the surface required for flow visualization is not a straight plane or not aligned with XYZ axes.
Though CAD clean-up process is done before meshing, some part of this depends on the type of mesh to be generated. In the next blog we shall know more about the step following CAD cleanup i.e. the factors governing the grid generation for CFD analysis turbomachinery.
The video below demonstrates the selection of the periodic domain and simplifying the geometry by selecting only the periodic part for grid generation process using the commercial tool ANSYS ICEM CFD.
- MIT course ware/multi stage axial compressor-http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node91.html
- An Introduction to Energy Conversion: Turbomachinery, Volume 3 by V. Kadambi, Manohar Prasad